The present application relates to data processing by digital computers, and more particularly to server virtual machines.
Some servers, such as enterprise servers or other large servers, can generally be characterized as request processing engines, serving large numbers of typically small user requests that belong to user sessions. The actual request processing involves running user code, such as Java servlets or Enterprise Java Beans (EJBs), in a virtual machine (VM) executing on a server. Scalability in such a server is traditionally achieved through the use of thread, i.e., a multi-threaded VM is used to process requests corresponding to a number of user sessions. However, system robustness requires strong isolation between user sessions, something that is often hard to achieve if a large number of user sessions are running within a single VM.
Operating systems can provide nearly perfect isolation for processes. In some operating systems, a crashed process will not affect other processes, and will not leak or leave behind allocated resources. It is conceptually possible to isolate user sessions and thus increase the robustness of a server by allocating one operating system (OS) process for each user session, and running a VM for that user session within the allocated process. Such an approach is not feasible, however, due to the OS scheduling overhead that would be incurred in switching between a large number of processes, and also due to the resources that such an approach would consume. OS processes are not designed to model entities as fine-grained as user sessions.